Mr. Dardicks Automatic Revolver. Fired a triangular cross section plastic cartridge (the tround), could switch from .22 to .38 by changing only the barrel. They also experimented with one that used a SALVO cartridge- four shots at once.

The MBA Gyrojet Rocket pistol. Each cartridge was a .45 caliber solid fuel rocket. Recoil of a .22, power of a 45, accuracy of a thrown chair. Ammo was $6 a bang in 1965. Today? Don't ask!